Purchase not completed

1. Tap Settings > iTunes and App Stores > Apple ID > View Apple ID enter your Apple ID and password then tap OK. View ‘Payment Information’ and make sure that payment account information is correct and up to date. Enter your security code and expiration date.
2. Restart your device
3. Log out of the iTunes store by going to Settings > iTunes & App Stores > Apple ID > Sign Out. Then sign in.
4. Go to Settings > General > Restrictions. If you enabled Restrictions previously, make sure that you are allowed to (a) access ‘iTunes Store’ and (b) make ‘In-App Purchases’. iOS 12 has changed the “Restrictions” settings. Now it is called Screen Time. Simply tap, Settings > Screen Time > Content & Privacy Restrictions > You will need to enter your password which is your old Restrictions password > iTunes & App Store Purchases and Allow “In-app Purchases” and “Installing Apps”.

If the above steps do not help, Get Support

Contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide 

Apple Support schedule phone call      

Starting an online chat with Apple Support 

If necessary, try Twitter

https://twitter.com/AppleSupport.          People have posted it may take an hour or more to get a response.

I have not seen a definitive solution for your problem. So, I suggest you use my search link in trying to solve it: puurchase not completed app apple. You will see a number of links that are, or may be, related to your specific problem. Several links related to my search claim fixes.

If you want to contact Apple Support to deal with your problem, see:

Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient. Sometime this site is not fully functional.

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter https://twitter.com/AppleSupport  There may be a wait of an hour or so.

Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best one.